1. Field of the Invention
This invention generally relates to a PCM Pulse Code Modulation signal processor and particularly to a PCM signal processor in which an analog signal such as an audio signal is digitized for recording and reproducing on a recording medium such as tape or a disk.
2. Description of the Prior Art
There has been proposed a PCM recording and reproducing apparatus for modulating an analog audio signal such as music on a PCM signal and to add an error correction code and an error detection code thereto so as to convert it to a signal configuration similar to a television signal, which is recorded and reproduced by a VTR (Video Tape Recorder). In the VTR, when dropout is caused by dust on the magnetic tape, which is the recording medium, there is a possibility that a burst error will occur in the recorded PCM signal. If such burst error occurs, it becomes quite difficult to error-correct the lost data.
To cope with this the following operation is performed. In a PCM processor wherein an audio PCM signal is converted to a quasi-video signal, upon converting of the audio PCM signal to the video signal, i.e., during recording, the PCM data is separated for each sampled word having a predetermined number thereby to form one block and an error correction code is generated for this one block of PCM data. The PCM data and the error correction code are respectively interleaved so as to have different delay times for each other and the error detection code is further added to the interleaved data, and signals are then converted to a video signal. When the VTR reproduces the signal thus recorded, the video signal is converted to the audio PCM signal in an operation opposite that described above, and de-interleaving is performed so as to disperse the burst error and to correct the data. According to such an arrangement, during normal reproduction, all correction capabilities are utilized so that nearly all of the original PCM data can be reproduced.
However, since the error detection function cannot detect or identify the error with absolute reliability, the data is supplied to an error correction circuit where it is not always regarded as erroneous. When the VTR has a dropout compensation circuit or the like it can replace a previous block data with the dropped data when the dropout occurs and is utilized by a recording and reproducing apparatus. A data array is converted to an original array by the de-interleaving during reproduction which may become different from the original data. When different data which has already been interleaved are connected to each other so they can be edited, different data may be mixed with the data to change it to the correct array by the de-interleaving. In these cases, since the error correcting circuit performs the error correction, the error correcting circuit produces data quite differently from the original data and when the data is supplied and converted to the audio signal by a D/A (Digital-Analog) converter, such audio signal may be heard as an offensive abnormal sound.
In order to prevent erroneous correction, a method has been proposed where the error decision is performed by employing both the error detection results using an error detection code and syndromes formed from the error correction code and the reproduced PCM data. That is, when an error syndrome indicates the presence of an error although an absence of an error is detected by error detecting means and when the syndrome decides on the existence of another erroneous word in addition to the erroneous word identified by the error indicator which is the result of the error detection and the position thereof is unknown, the error correction thereof is inhibited and instead, all PCM words within the block are compensated. For compensation, there is used a front-end hold to interpolate the erroneous word with the correct word occurring prior thereto and a mean value interpolation to interpolate the erroneous word with the mean value between the correct words before and behind thereof. Further, if such abnormal states occur in succession, the compensating operation becomes impossible so that muting must be applied.
The method thus described can prevent erroneous correction. In fact, to apply the muting causes the sound to be lost for some periods and when two system inputs are provided for the reproduced input which are switched or when a magnetic tape wherein data of two forms are connected and recorded and is reproduced by a PCM editing apparatus, although the number of errors in data detected by the error detecting signal is small there remains a drawback in that such erroneous data is detected as an abnormality caused by a mixture of different data into one block after de-interleaving and this results in muting which causes sound to be lost in the proximity of a connecting point of the data for a fixed time period.